The optical nonlinearity in bacteriorhodopsin (BR) is studied. The absorption spectra of BR before and after illuminating BR are measured respectively and the result shows a great change of absorption to light. Its optical nonlinearity by Z-scan is studied, the result of the theoretical calculation shows that it has bigger optical nonlinearity coefficient than other materials. A saturable absorption at wavelength 632.8 nm is found by measuring and theoretically simulating the change of its transmissitivity with input intensity.